(1) Field of the Invention
The present invention relates to a valve drive train for an internal combustion engine, particularly, for a V-type engine.
(2) Background of the art
A Japanese Patent Application First Publication No. Showa 60-164607 published on Aug. 27, 1985 exemplifies a method for adjusting the tension of a timing belt of an engine.
In a V-type engine in which two (left and right) cylinder banks (rows) are set at an angle, or V to each other and a camshaft is attached on an upper part of each cylinder row (bank). Due to the rotation of the camshafts, intake and exhaust valves installed on the respective cylinders are open and closed. A single timing belt (toothed belt) is wound around each cam pulley (toothed pulley) attached around an axial end of each cam shaft together with a crank pulley (toothed pulley) attached to a crankshaft.
The timing belt is driven in synchronization with the rotation of the engine.
In the above-identified Japanese Patent Application Publication, a cam structure of each cylinder row (bank) having the same phase and the same profile is used as well as each camshaft.
However, the rotation of one or the other of the camshafts is affected by, e.g., vibrations of the timing belt between the respective pulleys, vibrations generated around an axle of the crankshaft in a case where the camshafts of the left and right cylinder rows (banks) are driven by means of a single timing belt. Therefore, errors occur in the opening and closing intervals of the intake and exhaust valves of the respective cylinders along one of the cylinder rows (banks).
Belt tension between the crank pulley and front cam pulley and belt tension between front and rear cam pulleys are different from each other depending on the direction toward which the timing belt is driven to rotate. This creates vibrations of the timing belt as each cam pulley described above, i.e., follows different fluctuations and elongations of the timing belt.
The rotation of the camshaft to which the cam pulley is attached is affected and delayed.
FIG. 1 shows the result of an experiment with a six-cylinder V-type engine having each camshaft of the same profile and same phase. As shown in FIG. 1, the experiment indicates that at one of the cylinder rows (banks) (second, fourth, and sixth cylinders) in which the cam pulley was placed at the front side with respect to the driven direction of the timing belt, the opening timing interval of the intake valve on each cylinder (second, fourth, and sixth cylinders) and closing timing interval of the exhaust valve on each cylinder (second, fourth, and sixth cylinders) were delayed by .DELTA..theta..sub.i and .DELTA..theta..sub.e with respect to their respective design values .theta..sub.i, .theta..sub.e.
In this case, the delay quantity .DELTA..theta..sub.e of the closing timing interval of the exhaust valve is larger than the delay quantity .DELTA..theta..sub.i of the opening timing interval of the intake valve. This is, e.g., because resistance becomes large due to the overlaps of the closing timing intervals of the exhaust valves on one of the cylinder rows (banks) (second, fourth, and sixth cylinders) with the opening timing intervals of the exhaust valves on the other cylinder row (bank) (first, third, and fifth cylinders).
Hence, appropriate opening and closing intervals of the intake and exhaust valves on one or the other of the cylinder rows (banks) and a predetermined valve overlap cannot be achieved so that an engine performance will accordingly be reduced.